Preparation of Cu-mordenite by ionic exchange reaction under milling: A favorable route to form the mono-(μ-oxo) dicopper active species

Cu-exchanged mordenite samples were prepared using both solid state and aqueous solution ionic reactions. The obtained materials were then characterised from UV-vis, XPS, and TPR data in order to obtain information on the state of copper in the exchanged samples. Three types of copper species were identified, surface clusters of Cu₂O, hydrated Cu(II) ions with two slightly different coordination environments, and mono (μ-oxo) dicopper core, [Cu ₂O]²⁺, located in β type channels for samples activated on heating in the presence of oxygen. The formation of this last species, active for the methane conversion into methanol, was found to be particularly favored for the solid state ionic exchange reaction, and it is detected as an intense charge transfer band at 400 nm in the recorded UV-vis spectra and a well-defined peak at 936.34 eV of binding energy in the XPS spectra. The stabilisation of this dicopper core is the main distinctive difference between the exchange reactions in the solid state and in solution. The appearance of this bridged binuclear Cu²⁺ species was also detected in the recorded TPR profiles. According to the recorded CO₂ adsorption isotherms, the ionic exchange process modifies the accessible pore volume but the material preserves its porous features. The ability of that core for the low temperature methane conversion into methanol was confirmed recording chromatographic profile at different temperatures.